Sea Bed Logging methods can be used to detect hydrocarbon reservoirs beneath the sea floor. These methods comprise deploying an electromagnetic source near the sea floor and measuring the response using receiver instruments located at intervals spaced across the sea floor. The receiver instruments may be in the form of long rigid arms carrying electrical sensors, extending from a central body, which facilitates the detection of low level electrical signals in sea water.
The method of deploying existing instruments is as follows: the positively buoyant instrument and an attached concrete anchor are dropped from a survey vessel at a chosen location; the instrument sinks freely to the sea floor; the position of the instrument while sinking is monitored by acoustic methods; the instrument is positioned on the sea floor in a desired location when steady and the instrument is then used to measure and store data while located on the sea floor. After the measurements are complete, acoustic commands from the sea surface cause the instrument to be released from the anchor; the instrument then floats up to the sea surface for retrieval by a survey vessel and the data are extracted from the instrument.
The speed and efficiency of deployment and storage of the receiver instruments affects the overall efficiency of the survey. The mechanical performance of the instruments also affects the survey results. While instruments are known which may be partially disassembled for storage, these require sensor wires to be disconnected from sensors and data loggers when removing the arms for storage. This leads to the requirement for a system to track all the individual arms, sensors and data-loggers, and manage the reconnection back to the correct channel input. Further, a risk of failure or degradation is introduced each time a marine electrical connector is opened and reconnected.
Higher degrees of accuracy in sensor positioning are required for new acquisition techniques, for example, 3D acquisition, as well as being desirable for improving the accuracy of results obtained from other processing techniques. Therefore, there is a need to improve the performance of receiver instruments, in order to improve the accuracy and efficiency with which surveys may be carried out.